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BQ27426: choosing the termination voltage for the factory learning cycle

Part Number: BQ27426
Other Parts Discussed in Thread: BQSTUDIO, ,

Hi

Unfortunately we experienced sudden SOC drops (e.g. from 20% directly to 0%). When battery is nearly empty at 10% our application needs to do additional tasks before shutting down. However, with these sudden jumps we miss that point and run the system under the termination voltage which can lead to brown outs or resets. We therefore decided to do a learning cycle with the BQ27426EVM-738 and EV-2400 to hopefully improve fuel gauge accuracy. We were able to do an automatic learning cycle with bqStudio. However we are unsure which termination voltage we should use for the learning cycle. Our system termination voltage is 3.55V. The guide to achieve a successful learning cycle recommends to use the minimum specified voltage in the batteries datasheet (SLUA903 chapter 2.6). But the reference manual (SLUUBB0 chapter 7.4.3.1) mentions that the resistance profile has a higher resolution at 0% SOC. Therefore we assume, to improve gauging accuracy, it's better to also use the same termination voltage for the learning cycle.

Is that assumption correct?

Thanks and regards

  • A learning cycle should be run to cover as much depth of discharge (DOD) as possible, hence the minimum cell voltage for this chemistry is the proper cut-off point for a learning cycle.

    The gauge's built in cell model uses more cell resistance measurements for high DODs. SOC will be zero % at high DOD but it's usually not at DOD = 1.0 (DOD ranges from 0.0 to 1.0 with the gauge storing this as a scaled integer x16384) and certainly not at DOD = 1.0 for a terminate voltage of 3550mV (which is very high for the chemistries in the bq27426 and may cause additional issues). For a learning cycle, you want to have the gauge learn these cell resistances (measure them) for DOD all the way to 1.0 and this is only possible if you discharge to the minimum cell voltage for a chemistry (with a load that isn't too high, e.g. C/5).